DESCRIPTIONMaster designer and craftsman Jim
Kortge, K8IQY has developed a precision VXO (variable crystal oscillator) and the NJQRP Club is providing
it as a
kit including: all parts, a gorgeous red pc board, all board-mounted controls, a Ten-Tec
enclosure, and a very comprehensive construction and operating manual.

This kit
is useful in crystal measurement and sorting, as an extremely precise crystal
oscillator, as a way to bend a given crystal to place it directly on the desired
frequency, as a local oscillator in a radio, or as a direct conversion
transmitter unto itself.

The Precision VXO generator
employs a Colpitts oscillator using a JFET.The crystal is usually one from a set that will be used for building a
filter or for Local Oscillator service.However,
if the Precision VXO is being used for receiver alignment, the crystal might be
one whose frequency is inside one of the lower ham bands.Controlling the frequency of oscillation is accomplished with
a varicap diode and a set of 5 molded inductors configured as a binary weighted
set.Inductance needed to force the
crystal to oscillate at its marked case frequency is experimentally determined
by selecting the desired total inductance using five toggle switches mounted on
the board.When no inductors are
selected the crystal will oscillate at its highest frequency.As more inductance is added the crystal’s oscillating frequency moves
lower.Maximum inductance is obtained when all of the inductors are
selected and connected in series.Under
these conditions many higher frequency crystals may cease to oscillate.

Small frequency variations are
effected by means of a 10-turn potentiometer that varies the voltage applied to
the varicap diode. This fine tuning provides the overlap needed with most
crystals to provide seamless coverage for the digitally selected inductance.It can also provide sufficient frequency span so that the
resonant frequency of another crystal under test can be determined.Frequency precision is obtained by using the 10-turn pot and stability is
achieved through the use of an internal regulated supply, and by minimizing the
RF levels in the oscillator circuit.

A second transistor stage is
used as a source follower to further isolate the oscillator from any changes
reflected back from the output load.A
voltage divider is used to provide an output impedance of 50 ohms which properly
drives the fixed input impedance of a noiseless 2N2222 Norton amplifier. This
output stage
provides a gain of +12 dB and amplifies the generated signal to a level of
approximately –10 dBm, or 250 millivolts peak-to-peak.Downstream of this amplifier, a low pass filter is used to reduce
harmonic content above 20 MHz.A
spectrum plot of the generator’s output is shown in figure 2.

Overall, this Precision VXO generator is
designed to work with commonly available HC49 and HC49U style computer crystals,
often used in IF filter service. It will operate with crystals in the
frequency range of 3.5 MHz through 13.5 MHz, but is optimized for crystals at 9
MHz and below.

Besides having a clean output spectrum, this
low cost generator also has excellent frequency stability. Its warm-up
drift is a few Hertz and the stability over a 24-hour period is within 20 Hz.
Short-term stability is not measurable with the equipment in the K8IQY
laboratory.

The typical tuning range with a 4.9152 MHz
crystal is plus and minus 250 Hz - more than adequate for covering the series
resonant frequency of crystals being characterized or matched. More
importantly, with the wide range of inductance available, the oscillating
frequency of the VXO crystal can be moved over a considerably wider range,
thus ensuring that one can find the series resonant frequency of a crystal
under test.

The parts supplied in the kit
contain everything needed to assemble the unit except a power connector.(This part was not supplied, since
everyone seems to have his own favorite.) A
Ten Tec TP-17 case is included. The case is not drilled nor labeled - the
builder must perform those operations and templates are provided to assist with
those construction steps.

Assembly
is broken up into logical steps, so that each section can be built and tested.While that approach is recommended, it is not necessary, as the PC board
is logically laid out and well marked with the component locations.No alignment operations are required after assembly.

Note:
Enough interest was also shown in the companion "crystal test fixture"
that Jim used to sort the crystals for the 2N2-40 Group, and the NJQRP will also
be kitting that project in the near future in order to further enhance the
usefulness of this K8IQY-designed test equipment.

Friday night at the Ramada Inn during the vendors display I
purchased from the NJQRP Club the K8IQY 'Precision VXO' kit. For those
that have been hiding out in a cave, this is a VXO that allows to do a number of
things with crystals.

If you are an experimenter you can use this with some other equipment to measure
a number of crystal parameters or for a simple test application you can use it
to match crystals for a crystal filter. You'll see over the next few
months a number of individuals post some applications that they have found for
this kit.

I got mine built today and it worked from the start. I skipped all the
intermediate test stages, but you should not just in case.

The PC board is 10.2cm by 7.7cm and is neatly silk screened with a red board and
white lettering and parts outlines. Not only is the part number, e.g. C8,
shown but also the value to be used as a double check.

The manual says that you can do up to 13.5MHz but I find that I was able to use
crystals up to 18.0MHz. Above that the crystals that I have did not
resonant or show any activity at all, but 18.0MHz is good enough a it is rare to
find an IF frequency higher than 13.5MHz.

I think that the kit could even be built by a first timer even though the
construction notes are not step-by-step. You just need some basic ability to
tell the difference between a resistor and a molded inductor and a capacitor.

I do not know how many kits the NJQRP club plans to kit, but if you have not
gotten one I seriously suggest that you get one ASAP or we'll be seeing you post
to QRP-L later asking if anyone has one unbuilt. :-)

It comes with PC board, parts, and case. The only thing missing is the 12V
power jack and that you have to get according to the standard you have settled
on in your shack. You do have to drill the case and label it. No
serious metal work involved and the instructions have a template for this work.

Thanks to Jim Kortge, K8IQY, for the design and the NJQRP club for the kit.
This puppy is going to be one of the top test equipment devices in my lab for
sure.

I have no association with the club or this kit other than a satisfied and happy
customer. They do good work.

I will have an RF voltmeter circuit to be used with this puppy by Monday on my
web site along with digital pictures showing my layout and jig for testing
crystals. Previously I was using an expensive DDS VFO to do this work and for
those on a limited budget this is a much better way to go.

For more serious work later you are going to need something like the old Heath
IM-2410 Frequency Counter if you can find one for under $50 at a swap meet and
it is in working condition. I found one in good shape for $40 for Doug
Hendricks, KI6DS, at Dayton years ago and Doug can tell you how valuable that
puppy is......